This invention relates to electrochemical converters employing solid oxide electrolytes and methods for making the same, as well as assemblies employing such components or methods.
Electrochemical converters perform fuel-to-electricity conversions in a fuel cell (electric generator) mode and electricity-to-fuel conversions in an electrolyzer (fuel synthesizer) mode. The converters are capable of high efficiency depending only on the relation between the free energy and enthalpy of the electrochemical reaction. The converters are not limited by Carnot-cycle considerations.
The key components in an electrochemical energy converter are a series of electrolyte units onto which electrodes are applied and a similar series of interconnectors disposed between the electrolyte units to Provide serial electrical connections. Each electrolyte unit is an ionic conductor with low ionic resistance allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operating conditions of the converter.
It is known that solid oxide electrolytes, such as zirconia, stabilized with compounds, such as magnesia, calcia, or yttria can satisfy these requirements when operating at high temperatures, e.g., about 1,000.degree. C. These electrolyte materials utilize oxygen ions to carry electrical current. The electrolyte should not be conductive to electrons which can cause a short-circuit of the converter. On the other hand, the interconnector must be a good electron conductor.
The interaction of the reacting gas, electrode, and electrolyte occurs at the electrode-electrolyte interface which requires that the electrodes be sufficiently porous to admit the reacting gas species and to permit exit of product species.
Electrochemical converters are further described in U.S. Pat. Nos. 4,614,628; 4,629,537 and 4,721,556, all of which are hereby incorporated by reference. In particular, U.S. Pat. No. 4,614,628 discloses solid oxide electrolyte structures and methods of their formation. According to this reference, such electrolyte structures are prepared by (1) forming a solid oxide electrolyte layer upon a substrate by plasma deposition; (2) removing the solid oxide layer from the substrate; (3) sintering the solid oxide layer; (4) depositing a fuel electrode material on one surface of the sintered solid oxide layer; and (5) depositing an oxidizer (or air) electrode material on a second surface of the sintered solid oxide layer.
Although the structures formed according to this method are satisfactory, the step of sintering involves additional cost and is time consuming. With this method, there is also the disadvantage of relatively low yields due to possible damage to the electrolyte in handling it during processing following step 2. Similar problems are encountered when manufacturing interconnector plates, particularly when conductive ceramic materials are used. It would, thus, be desirable to provide an alternative method of modified procedures and/or sequences for forming such structures.
Accordingly, it is an objective of the invention to provide more economical and reliable methods of manufacturing solid oxide electrolyte and/or interconnector structures for use in electrochemical energy converters. It is also an objective of the invention to provide methods which minimize the risk of damage to or destruction of the plates during their formation. These and other objectives of the invention will be apparent to one skilled in the art from the disclosure which follows.